1. Field of the Invention
The present invention relates to a recording/reproduction apparatus, recording/reproduction method, program, and recording power adjustment apparatus capable of adjusting a recording power (e.g., a write power, an erase power, or a bottom power). More particularly, the present invention relates to a recording/reproduction apparatus, recording/reproduction method, program, and recording power adjustment apparatus capable of adjusting a recording power based on a partial response maximum likelihood (PRML) technique which is a type of reproduction technique.
2. Description of the Related Art
Phase change optical discs (CD-RW, DVD-RAM, DVD-RW, Blu-ray Disc, etc.) are known as rewritable optical discs. For phase change optical discs, multipulse laser light is used to perform overwrite recording. Laser light has, for example, a write power Pw, an erase power Pe and a bottom power Pb.
FIG. 24 shows a multipulse having a write power Pw, an erase power Pe and a bottom power Pb.
The write power Pw is used to change the state of a recording film from a crystal state to an amorphous state so as to form a mark. The erase power Pe is used to change the state of the recording film from the amorphous state to the crystal state so as to erase (overwrite) an old mark. The bottom power Pb corresponds to a power of a bottom portion of a multipulse, and is used to prevent thermal diffusion due to irradiation of laser light during recording using a multipulse.
Conventionally, in recording/reproduction methods and recording/reproduction apparatuses for recording digital information onto a recording medium, a test signal is recorded onto a recording medium while changing the write power Pw, the erase power Pe and the bottom power Pb in various manners, and the recorded test signal is reproduced. A predetermined signal evaluation index is detected to determine whether or not the recorded state of the signal is normal. In order to achieve the recorded state which allows an optimum or desired state of a predetermined signal evaluation index, the power of laser light is controlled and determined. Examples of the predetermined signal evaluation index include jitter, asymmetry, an error rate (a bit error rate (BER)), the degree of modulation, and the like (e.g., Japan Patent No. 3259642 (particularly, FIG. 1)).
Alternatively, a method for controlling and determining the power of laser light by referencing a signal evaluation index based on a PRML expected value error has been proposed. For example, Partial Response Maximum Likelihood (PRML) is a signal processing technique which can be expected to improve reproduction performance when original digital information is reproduced from a recording medium. PRML is a combination of waveform equalization called PR and maximum likelihood decoding called ML.
Conventionally, the characteristics of a recording/reproduction transmission path are evaluated based on jitter in a binary pulse and a reproduced clock. However, it has been difficult to evaluate and optimize the characteristics of a reproduction transmission path based on the PRML technique. This is because jitter is not correlated with the performance (BER) of the PRML technique.
For example, Japanese Laid-Open Publication No. 2003-141823 (particularly, page 79, expression (14); page 173, and FIG. 14) discloses a technique in which an expected value error correlated with the BER of the PRML technique is used as an index instead of jitter. This index is used as an index to indicate the probability of occurrence of error due to stress in a reproduction system, such as a focus offset, a tilt or the like. The index is also used in best focus search or the like. In other words, the index is used to optimize a parameter which determines whether or not a reproduced state is normal.
However, in a method and apparatus which determines an optimum recording power using asymmetry as an index, the optimum power may not be obtained due to insufficient precision in detection of asymmetry. In a method and apparatus which determines an optimum recording power with the PRML technique using jitter as an index, the optimum power may not be correctly obtained, since a recording power minimizing jitter is not necessarily equal to a recording power minimizing BER. In a method and apparatus which determines an optimum recording power with the PRML technique using BER as an index, the optimum power may not be obtained due to insufficient precision in detection of the index. The insufficient precision of detection of the indexes is attributed to the requirement of a large quantity of recording area for measurement of BER, the degradation of BER due to stain, dust or the like on a disc rather than a recorded state, a change in BER (sensitivity) to a recording power due to the high error correction capability of the PRML technique, or the like.
As described above, when a conventional method of setting a parameter for a recording power in a manner which optimizes jitter, asymmetry or BER (e.g., Japan Patent No. 3259642) is applied to a system which employs the PRML technique, the probability of occurrence of error is not necessarily minimized. In addition, a recording power is not necessarily determined with good precision due to insufficient precision of detection of an index for determining a recording power. Therefore, a degradation in performance occurs due to cross power (overwrite is performed on a recorded area of a disc under different conditions), which is generated due to error in setting a recording power. Therefore, it may be difficult to obtain the stable compatibility of optical disc drive apparatuses and optical disc media to the same standard.